The present invention relates to a method of separating an aromatic or aromatic compound from an entry hydrocarbon mixture of arbitrary aromatic composition, especially an entry hydrocarbon mixture which can also contain nonaromatic components including paraffin, cycloparaffin, olefins, diolefins and organic sulfur compounds.
A method of separating an aromatic from an entry hydrocarbon mixture of this type is known. In this known method the separation occurs by an extractive distillation, in which N-substituted morpholine, whose substituents do not have more than seven carbon atoms, is used as selective solvent. The nonaromatic components of the entry hydrocarbon mixture are distilled off the head or top of an extractive distillation column used, while the aromatic components are drawn together with the selective solvent from the sump of the extractive distillation column and subsequently are separated from the solvent by distillation in a separator column connected to the extractive distillation column.
The above-described method of obtaining aromatic compounds has been known for many years and has proven satisfactory in the intervening time in a number of different large scale plants, particularly in the case in which N-formyl morpholine is used as a selective solvent. Normally the solvent drawn from the sump of the separator column is fed back into the extractive distillation column for reuse after suitable cooling. Up to now the solvent has been returned to the extractive distillation column at its head, because of basic process engineering considerations. Because of that the top product obtained contains a certain amount of residual solvent. This residual solvent content can amount to up to 2% by weight. Because of efficiency considerations and to obtain a top product, which is as pure as possible, it is essential that as much as possible of this solvent in the top product must be recovered.
Up to now it was standard practice that top product drawn as head product from the extractive distillation column be conducted into a separate distillation column, in which the hydrocarbons of the top product were separated from the solvent. Since the hydrocarbons of the top product must have a solvent content less than 1 ppm, this distillative separation requires a highly expensive apparatus (i.e. a separator distillation column with a high plate number) and a high energy consumption.
To decrease the high energy requirement, in German Published Patent Application 34 09 030 it has already been suggested that the distillative separation of the top product from the extractive distillation column be conducted under conditions such that the sump product obtained has a solvent content of from 20 to 75% by weight. Subsequently this sump product is separated in a separating vessel into a light phase and a heavy phase. The solvent rich heavy phase is returned to the extractive distillation column and the solvent poor light phase is fed to a raffinate distillation column. With this method of course the energy requirements for the purification of the top product are reduced. However, this still requires a separate column for the distillation of top product and also a separating vessel for the separation of heavier and lighter phase, which also means a not inconsiderable apparatus expense.